Crosstalk is the unwanted coupling of signals from one pair to another along a multi-pair cable. When crosstalk is measured at the same end of the cable where the crosstalk originates, the measurement technique is called near end crosstalk (NEXT). Twisted pair LAN technologies, such as 10BASE-T, 100BASE-T, and Token Ring are primarily vulnerable to cable crosstalk problems in a manner that is best tested by measuring the NEXT of the installed cable.
When crosstalk is measured at the end of the cable opposite from where the crosstalk originates, the measurement technique is called far end crosstalk (FEXT). FEXT is measured by applying a test signal to a wire pair at a far end of the cable and measuring the disturbance on the other wire pairs in the cable at the other or near end. It is relevant to specify the FEXT performance of cabling for network technologies, such as the new 1000BASE-T specification, that transmit simultaneously on multiple wire pairs in the same direction.
While it is easy to measure the FEXT performance of an installed multi-pair cable, it is difficult to specify certification limits for such measurements, since FEXT varies with the cable length. The equal level far end crosstalk (ELFEXT) measurement technique was developed as a practical alternative for field certification. Generally, ELFEXT equals FEXT minus attenuation caused by the cable. ELFEXT measurements compensate for the effect of varying cable length so that all installed cable can be certified to the same limit.
Residual crosstalk is any signal that is due to the test instrument itself. Residual crosstalk error must be taken into account in crosstalk measurement analysis.
With vector measurements, residual crosstalk errors can be mathematically cancelled. When an error term cancellation in a vector measurement is employed, the test instruments at either end of the cable need to generate test signals that are phase coherent with one another.
Accordingly there is a need for a method of generating phase coherent test signals at either end of the cable. There is also a need for a test instrument that can be used at either end of the cable and that is capable of producing a test signal that is coherent with another test signal generated at the other end of the cable.